The present invention relates to a router apparatus used in an IPv6 multihome network, a route information distributing method implemented by the router apparatus, and a communications system including the router apparatus.
Multihome environments are structured in IP networks. In an IP multihome network, an end site, such as a local area network of a company or a university, is connected to multiple Internet service providers (ISPs). By establishing connection to multiple upstream resources, load distribution is guaranteed and the reliability is improved. An example of such a multihome technique is disclosed in RFC (Request for Comments) 2260 and RFC 3178 documented by the Internet Engineering Task Force (IETF), which is a private organization for promoting standardization of Internet techniques.
The Internet Protocol version 6 (IPv6) address has a bit length four times as large as the conventional IPv4 address, and a global IP address can be assigned to a number of machines and pieces of equipment. In IPv6, Internet service providers (ISPs) assign 48-bit prefixes to an end site, which is a site of an end user or a subscriber contracted with the Internet service providers, unlike the IPv4. The site assigns 64-bit prefixes to host apparatuses arranged in that site.
FIG. 1 is an example of the conventional IPv6 multihome network. Site 100 is connected to two Internet service providers (ISP_a and ISP_b) 110 and 120. A host apparatus 50 located in the site 100 is connected to the Internet service providers (ISP_a and ISP_b) 110 and 120 via a router (not shown).
The host 50 has the prefixes assigned by the Internet service providers (ISP_a and ISP_b) 110 and 120. The prefixes are, for example, ISP_a 2002:1000:1000::/48, and ISP_b 2002:2000:2000::/48, respectively, where “/48” represents the IPv6 prefix length. The prefix is divided every 16 bits, and expressed as a hexadecimal value using colons.
FIG. 2 shows the IPv6 address structure in the site. The IPv6 address consists of a 48-bit prefix 151 assigned from an Internet service provider (ISP), a 16-bit subnet ID 152 assigned to a LAN in the site, and a 64-bit host address 153 for identifying each host apparatus used in the LAN.
The host 50 in the site 100 has multiple IP addresses with different prefixes, as illustrated in FIG. 1. In this example, the host 50 has the following IP addresses:                FECO:1000:1000:0001:0011:2233:4455:6677,        2002:1000:1000:0001:0011:2233:4455:6677, and        2002:2000:2000:0001:0011:2233:4455:6677.The IP address starting from “FECO::” is a site local address given by a site manager, which is unique within a specific area defined as “site”.        
In the IPv6 multihome environment, the number of bits of the prefix assigned by an Internet service provider (ISP) to a site is fixed at 48 bits, such that address modification becomes the minimum when the site changes the connected ISP.
In many sites, each router creates a routing table using a routing protocol, such as RIPng (Routing Information Protocol next generation) defined in RFC 2080 or OSPF (Open Shortest Path First) defined in RFC 2740. All the packets addressed outside the site are transmitted collectively using a route called a “default route” having a length of 0 bits (0.0.0.0/0) because these packets are simply supplied to a gateway (GW) router, which is a connection node between the multihome site and an Internet service provider. The default route is a general-purpose route used when no route information is found.
An ordinary in-site route is generated by the adjacent router of this link, and delivered in the site. In contrast, the default route is created by a gateway provided at the connection node between the site and the ISP, and delivered to each of the routers located in the site using a routing protocol. Upon receiving a packet, each router checks the destination address. If there is a corresponding route, that is, if the destination address is in the site, then the packet is transmitted in that direction. On the other hand, if there is no corresponding route, that is, if the destination address is outside the site, then the packet is output to the default route.
In this manner, each gateway (GW) router connected to an associated ISP creates a default route for in-site routing. With the current IP routing protocol, an appropriate route is determined based on distance when multiple default routes exist. Accordingly, a packet addressed to outside of the site is supplied to the nearest gateway router located closest to the sender.
Meanwhile, RFC 2267 provides an ingress filter, which checks the source address of a packet transmitted from a site and rejects the packet if the packet does not use the address prefix given by the ISP. According to this policy, if a packet is supplied through the in-site default route to a gateway router that does not correspond to the prefix, this packet is discarded by the ingress filter at the ISP.
FIG. 3 illustrates the packet filtering. In the site 100, a gateway router a (GWa) 51, which is a connection node to ISP_a 110, and a gateway router b (GWb) 52, which is a connection node to ISP_b 120 are provided. A router Ra 53 is connected to the gateway router a (GWa) 51. A router Rb 54 is connected to the router Ra 53 and the gateway router b (GWb) 52. A host 50 is connected to the router Rb 54.
The host 50 can generate a source address using a prefix 2002:1000:1000::/48 assigned the ISP_a 110, which is referred to as address A, and another source address using a frefix 2002:2000:2000::/48 assigned by the ISP_b 120, which is referred to as address B. The host 50 can set one of the addresses A and B as the source address when transmitting a packet.
When a packet is transmitted from the host 50 to outside of the site 100, the packet is first received at the router Rb 54, and supplied to the gateway router b (GWb) 52 through the shortest default route (with distance 1 in this example). If, in this case, the prefix 2002:1000:1000::/48 assigned by ISP_a 110 is used in the source address, this packet is discarded by the ingress filter of ISP_b 120.
To avoid this problem, RFC 3178 provides a technique for preventing undesirable packet from being discarded at the ingress filter, as illustrated in FIG. 4. With this technique, negotiation is manually made between a multihome site and an ISP, and static IP tunnels are provided between the gateway router a (GWa) 51 and ISP_b 120, and between the gateway router b (GWb) 52 and ISP_a 110. The gateway routers 51 and 52 check the source address of a packet to send this packet to the correct ISP.
JP 2002-359638A discloses a router system capable of automatically setting a temporary default route upon linking up of the port when activating a router. This technique can improve the reliability of switching operations of a routing table.
The technique using the static IP tunnel defined in RFC 3178 has a problem in that a redundant route may be generated. FIG. 5 illustrates an example of such a redundant route. When a packet is transmitted from the host “a” (50) to host “b” (60) located outside the site, the packet may be delivered through the IP tunnel connecting the gateway router b (GBb) 52, ISP_b 120, and ISP_a 110.
To be more precise, if the host a (50) transmits a packet to host b (60) using the prefix assigned from ISP_a 110 as the source address, the packet is first sent to the gateway router b (GWb) 52 located nearest to the destination address. This packet is further supplied to ISP_a 110 via ISP_b 120 through the static tunnel.
This means that the packet is transmitted through a redundant route, instead of using the optimum (shortest) route (extending via ISP_b 120, ISP_c 130, and the Internet 200 to the host b (60) in this example).
Using a static tunnel is also disadvantageous in a dynamically changing environment, such as in multihome mobile networks. Whenever the connection point to the ISP changes, the gateway address and the address prefix of the multihome site change, which makes it difficult to maintain the IP tunnel provided between the ISP and the gateway. No technique has been proposed so far to automatically supply such a dynamic change in address prefix to the router in the site.